Casting molds as used in low-pressure casting, gravity casting, squeeze casting or pressure-die casting, are usually made of hot-work steels because the recrystallization and/or transformation temperatures of these steels are distinctly above those of the molten light metal materials. In the casting processes, in order to obtain smooth surfaces on the cast components to be produced, it is required that the liquid melt, for example, in the form of a light metal alloy such as an aluminum alloy, will not adhere to the surface of the casting mold. For this purpose, the surfaces of the casting molds are provided with release agents or with facings to prevent the molten metal from sticking to the casting mold.
To ensure that the release agents and respectively the facings adhere to the tool surfaces, the latter first have to be cleaned and, in the given case, be passivated.
By passivating, a non-metallic protective layer is generated on the metallic material in order to slow down corrosion or to prevent corrosion as much as possible. In this regard, passivating by phosphating is of special importance. Phosphating is a widespread method of surface technology wherein, by a chemical reaction between the metallic surface of the workpiece and an aqueous phosphate solution, a conversion layer of tightly adhering metal phosphates is formed. Phosphating serves to protect from corrosion and generate a diffusion barrier. Additionally, it is thus possible to enhance adhesion, for example, in case of subsequently applied layers, and to reduce wear.
For phosphating, use is made both of phosphate baths and of phosphate spray systems. In both cases, it is required to clean the surface of the casting mold or of the workpiece prior to phosphating.
The cleaning process is performed, for example, by use of a high-pressure water jet which, via a rotating nozzle, is directed onto the workpiece at a pressure ranging from 1750 to 3000 bar. Disadvantageous herein is that the water contact of the cleaned workpiece causes corrosion and that organic and inorganic residues from the jet water remains on the surface. The high pressures leads to massive wear of the pistons and valves of the water-jet system and incurs high costs.
For this reason, cleaning processes performed at lower pressures of, for example, 200 bar are also known. Although these processes can be carried out with reduced wear, the cleaning effect deteriorates correspondingly.
It is also known to perform the cleaning of pressure die casting molds by uses of granulates which are blasted under pressure onto the workpiece. Herein, use is made, for instance, of nutshells or glass pearls. For the cleaning of low-pressure casting molds or gravity casting molds, the granulate used can also be provided in the form of steel, corundum or ceramics. Apart from an additional increase of mechanical wear of the surface, undercut portions of the treated component will be partly inaccessible. This gives rise to dimensional inaccuracies in subsequent casting processes and to impurities on the surface of the mold due to coating with foreign particles from the cycle.
When depositing a facing, for example, with sodium silicate binders, subsequent to such a cleaning process, the surfaces, which for the above-mentioned reasons have been insufficiently cleaned and passivated, will cause adhesion problems, giving rise to lattice defects on the surface of the facing after deposition. Particularly during a subsequent thermal treatment, a danger exists that the facing will peel off from the treated surface, or in casting molds, during subsequent casting processes, there is a danger of intermetallic welding on the lattice defects so that the mold cannot be accurately separated from the cast workpiece.
When using known cooling/separating agent systems for pressure die casting molds, problems also exist in the wetting of insufficiently cleaned surfaces or corroded surfaces of the mold. In the casting process, this will also cause intermetallic connections on the surface of the mold.
To avoid these advantages, it is thus necessary to perform a post-cleaning on the surface of the mold to obtain a metallurgically pure surface.
Known methods for cleaning and passivating are usually carried out in baths or by spraying treatment.
When the treatment is performed in a bath, the casting mold or the workpiece will, after the jet treatment, first be immersed into a pickling bath for removal of organic residues and oxides at temperatures from 40°-90° by means of inorganic acids and suitable surface-active agents. This process is followed by a deep cleaning process in the bath by ultrasonic means, whereupon the workpiece or the casting mold will be immersed into a further bath for rinsing and neutralizing. Subsequently, the workpiece must be dried and, in a further process step, be activated in the bath, before the phosphating is performed, for example, by means of zinc phosphate at 40-70° C. or manganese phosphate at 70-90° C. The workpiece or the casting mold are thereafter neutralized and dried. A disadvantage of these processes consists in the required long dwelling times in the baths, especially in case of large components such as pressure die casting tools. In correspondence thereto, large amounts of energy are needed for reaching and maintaining the required temperatures. Maintaining the clean condition of the bath in order to maintain the necessary bath parameters is also very burdensome because, between the individual baths, impurities will be generated, making it necessary to remove accumulating residues. Depending on the dimensioning of the components, the size of the base may also have to be adapted.
In spray treatment, the pickling bath is followed by a high-pressure cleaning process and then by rinsing and neutralizing with a suitable spray solution. After the subsequent drying and heating of the component, a spray activation is carried out at increased temperature before the phosphating is performed by means of a heated spray solution at 40-70° C. in case of zinc phosphate, and at 70-90° C. in case of manganese phosphate. This is also followed by the further steps of neutralizing and drying the workpiece or the casting mold. Similar to the treatment in a bath, the spray treatment also entails a relatively high energy consumption for reaching the required temperatures, particularly in case of correspondingly high mass ratios, so that the method is economically disadvantageous. There also exists a high logistic expenditure in the treatment cycle of the components to be treated.
Further still, the components treated with known passivating agents often suffer from an insufficient thermal shock resistance which is caused particularly by lattice defects in the structure of the passivating layer.
To improve the above situation, DE-34 03 660 A1 describes a passivating agent consisting of an aqueous solution of aluminum hydrogen phosphate and organic polymers which form a film under thermal influence. As organic polymers, use is made herein of acrylic or epoxy resins. When heated, however, these lacquers will lose their organic components. A special disadvantage of this agent resides in that, in case of several casting processes, lattice defects will be caused, entailing the risk of welding connections to a cast component. The thermal shock resistance is still also insufficient.